Migrate boxAgainstJava tests to multi-file framework

compiler/testData/codegen/boxAgainstJava/innerClass/kt3812.kt

@@ -1,5 +1,18 @@
+// FILE: Foo.java
+
+public class Foo {
+    public class Inner {
+
+    }
+
+    public class Inner$ {
+    }
+}
+
+// FILE: 1.kt
+
 fun box(): String {
-    kt3812().Inner()
-    kt3812().`Inner$`()
+    Foo().Inner()
+    Foo().`Inner$`()
     return "OK"
-}
+}